Process and product of metallurgically joining zirconium to ferrous metal



J. L. KLEIN ETAL R CT OF METALLURGICALLY JOINI IRC UM TO FERROUS METALJuly 7. 1964 y PROCES 4 sheets-sheet `2 Original Filed May 25 S AND P mmvm INVENTOR` Loewe/Islam, /f/e/o BY Kauf/afm Altar/leyV JUlY 7. 1964 J.L. KLEIN ETAL 3,140,108

PROCESS AND PRODUCT 0F' METALLURGICALLY JOINING ZIRCONIUM TO FERROUSMETAL Original Filed May 25, 1960 4 Sheets-Sheet 3 j l', i l\ a I *il y"Il .il *l f H INVENTOR.; Laewens/em, /rle/'l/ BY Kauf/nam Allamey- July7 1964 .J.1 KLEIN ETAL 3,140,108

PROCESS AND PRODUCT OF METALLURGICALLY JOINING ZIRCONIUM TO FERROUSMETAL Original Filed May 25, 1960 4 Sheets-Sheet 4 Alfa/nay- UnitedStates Patent O 3,140,108 PRQCESS AND PRODUCT F METALLURGICALLY JOININGZlRCONIUM TO FERRGUS METAL Joseph Lester Klein, Arlington, Albert R.Kaufmann, Lexington, and Paul Loewenstein, South Lincoln, Mass.,assignors, by mesne assignments, to the United States of America asrepresented by the United States Atomic Energy Commission Continuationof application Ser. No. 31,786, May 25, 1960. This application July 28,1960, Ser. No. 46,041 4 Claims. (Cl. '287-119) The present inventionrelates in general to the provision of metallurgically bonded jointsbetween zirconium or high-zirconium alloys and ferrous metal. Moreparticularly, it relates to an improved method and resulting product ofjoining such predominantly-zirconium metal to stainless steel,especially austenitic stainless steel, in the form of rods, tubes, andthe like. As is known, metallic zirconium and high-zirconium alloys havebecome quite promising as specialized materials of construction. In thechemical processing industry, their excellent resistance to corrosion bysubstantially all dilute mineral acids, hot concentrated aqueouscaustic, and high temperature pressurized water, and their furthergeneral ability to withstand nitric acids in hot concentrated solutions,are markedly advantageous for service as inert process piping. Moreprominently in the nuclear energy art, zirconiums exceptionally lowneutron absorptivity combined with strength at elevated temperatures hasmade it outstanding for mechanical structure directly Within the chainfission reactive cores of high-temperature thermal power-productiveneutronic reactors. In fact, the designs of several such neutronicreactors currently being engineered for construction each employ abundle of elongated, co-extensive, spaced-slightly apart, re-entrant,zirconium alloymore particularly Zircaloy-Z-tubes as the principalpressure-retentive structure of its core; a multiplicity of fissionablefuel elements arrayed within the essentially neutron transparent tubescooperatively affords self-sustained chain fission reaction, whereuponcirculating pressurized coolant fluid conducted through the tubes inheat-transfer relationship with those elements removes useful heatgenerated.

As a cardinal impediment in such applications, though, a series impassehas heretofore been encountered in making any satisfactory unitary jointbetween predominantlyzirconium metal and ferrous metal, especiallystainless steel. It is now rather generally accepted in the art thatfusion welding, by any of the familiar techiques, is ineffectualtherefor; resulting weldments have proven to be, at best, very poor inmechanical properties and corrosion resistance. The want of such a jointhas been particularly adverse in the case of zirconium pipes and tubes,Where connections to ferrous metal vessel nipples, tubing runs, and thelike must often be absolutely uid-tight in view of the extra hazardousradioactive or corrosive character of the fluids carried. Variousmechanical jointsgasketed couplings, rolled and other frictionalconnections, and the like--investigated have, in common, left much to bedesired; leakage, albeit sometimes only temporary, attending temperatureexcursions, especially upon rapid heating or cooling, stands as anotable shortcoming. Indeed, in said contemporary power-productiveneutronic reactor design, the need for the advent of some reliable,unitary joint to so couple the ends of each in-core zir- ICC coniumpressure tube to contiguous stainless steel extension tubes outside thereactors core region has become crucial to the basic technicalfeasibility of the pressure-tube-type reactor.

Accordingly, an object of the present invention is to provide a rigid,strong, unitary metallurgically-bonded joint between zirconium as Wellas high-zirconium alloys, and ferrous metal, especially austeniticstainless steel.

Another object is to provide such a joint particularly suited forbuttwise connection of pairs of members both similarly in the form ofrods, tubes, or like configurations of substantially uniform axialcross-section.

A further object is to provide such a joint particularly suited tojoining contiguous tubes in a reliably fluid-tight relationship.

Still another object is to provide such a tube joint which does notrequire encircling outside collars, inside sleeves, or other bulkymember either exceeding the outside diameter of the tubing orobstructive of the tubing interior.

Still a further object is to provide an improved method of fabricatingsuch joints.

Yet another object is to provide such a method readily and simplyadaptable to fabricating such a joint integrally and simultaneously withthe fabrication of members of substantially uniform axial cross-sectionto be joined.

Yet another object is to provide a method adaptable to fabricating acontiguous series of such joints as a single product readily severableinto individual unitary nipples each featuring one extremity of thepredominantly-zirconium metal and the other extremity of the ferrousmetal.

Again another object is to provide such a method of appropriate fitnessand suitability for large-scale metalworking application.

An especially important object is to provide such a joint which ismarkedly corrosion resistant.

Other objects will become apparent hereinafter.

In accordance iwth the present invention, a metal constitutedpredominantly of zirconium is metallurgically joined to a predominantlyferrous metal by arraying in tandem a mass of said predominantly ferrousmetal followed by a substantially contiguously abutting mass of saidpredominantly zirconium metal both within a susbtantially-vacuum-tightmalleable ferrous metal can, and, while establishing and maintainingsubstantial evacuation of gas from the interior of said can, hotextruding said enveloping can and concomitantly, in axial tandem, thetherein contained said ferrous mass followed by said contiguouslyabutting predominantly-zirconium mass, such arraying and extrusion beingeffected after rst degassing said malleable ferrous metal can and allother metal which is afforded communication with said zirconium metalduring hot extrusion and contains nitrogen subject to additionalsusbtantial evolution in vacuo at the temperature of extrusion, saiddegassing being effected by protractedly heating without melting andconjointly drawing a substantial vacuum thereupon. Upon said extruding,the leading face of the zirconium billet penetrates as a slender wedgedeepely into the trailing face of the ferrous mass, affording a soundlymetallurgically bonded juncture of extensive area and high strength.Typically, upon so extruding, in tandem, billets of degassed type 304L,347, or 321 stainless steel followed by Zircaloy-2 with planar matingfaces at about 1600-l650 F. and 6:1 cross-sectional area reduction ratiowith about several feet per minute ram speed, all within a degassed 16gage SAE- 1015 can, extruded rods result having conical interfacialjunctures between the two metals of slender taper of axial lengthapproximating five times the rod diameter; across the juncture themetals prove to be soundly bonded with manifested tensile strengthnormal to the interface of the order of 35,000 to 55,000 p.s.i.-is somecases greater than 60,000 p.s.i.-and similar strength in shear along theinterface. This compares quite favorably with the about 70,000 p.s.i.ultimate tensile strength of the Zircaloy-2 itself.

As applicant has discovered, the crucial key to this unique realizationof metallurgical bonding is the rigorous degassing of the ferrous metalca n and also every ferrous metal billet subject to deleterious nitrogenevolution upon being heated to the extrusion temperature. The ferrouscomponents are heated and held for an extended periodsay severalhours-to a temperature best higher than the hot extmsion temperatureunder a good dynamic vacuum. Merely maintaining a vacuum upon theassembled billets during extrusion proves inadequate. Indeed, withoutsuch preliminary protracted hot degassing, the tandem extrusionoperation, even though conducted in vacuo, exhibits much the sameunsatisfactory results as have characterized previous fusion weldingattempts; representative tensile strengths of the poor order of 6,000 to9,000 p.s.i. are suffered. It is thought that the zirconium component,upon heating, tends to act as a getter in removing residual traces ofelemental nitrogen from the abutting heated ferrous metal and can, andthe consequent elevated-temperature nitriding of zirconium at thezirconium-ferrous interface is a prime adverse mechanism to which theunsound bonding otherwise experienced is attributable. Thus, tenaciousnitrogen in the ferrous components, albeit in minute amount, is regardedas the agent primarily obstructive to bonding. In the instant process,not only does the critical hot degassing evidently purge the ferrousmetal effectively, but then the extrusion operation, propitiously beingof a mechanical nature admissive of maintaining an evacuated conditionthroughout, is seen to cooperate by affording substantial exclusion ofsuch obstructive agent during the entirety of hot joining. However, inview of uncertainty respecting the complete mechanism responsible forthe sound bonding achieved, it is not intended for this invention to belimited to any particular theory as to the precise phenomena involved.

The particular zirconium materials amenable to the instant process aresubject to wide variation. Both principal types of elemental zirconiumof current significance in the art-viz., raw zirconium metal produced bymagnesium reduction of zirconium tetrachloride (i.e., Kroll Process),and refined, crystal-bar zirconium derived by hot-wire decomposition ofzirconium iodide vapor (i.e., van Arkelde Boer Process)are suitable. Inaddition, applicability extends generally to alloys constituted mostlyof zirconium; typical are the binary alloys of the order of 0.5 to 2.0weight percent of such metals as aluminum, chromium, copper, hafnium,iron, manganese, molybdenum, nickel, niobium, tin, titanium, tungsten,and vanadium. Further, it has been notably successful in the case ofthose multicomponent high-zirconium alloys which now stand as the formsof zirconium of primary commercial importance for structural service,particularly the ductile, corrosionresistant Zircaloy-2: zirconium plusapproximately 1.2 to 1.5% (by weight) tin, 0.07 to 0.2% iron, 0.05 to0.15% chromium, and 0.03 to 0.08% nickel, and the alsohydrogen-embrittlement-resistive Zircaloy-4; Zr plus approximately 1.2to 1.5% Sn, 0.07 to 0.2% Fe, and 0.05 to 0.15% Cr. Likewise, diversetypes of extrudible ferrous metal may be used. In addition to mild andcarbon steels and myriad steel alloys, the stainless steels, bothferritic and austenitic along with martensitic, are suitable. Amongthese, stainless steel types 347, 321 and 304L (American Iron and SteelInstitute type numbers and compositions) all austenitic with basiccomposition of about 18% Cr plus about 8% Ni-have been found quitesatisfactory. It is desirable, of course, that the selected ferrousmetal should not be subject to deleterious metallurgical reaction underthe degassing and extrusion conditions employed. For instance, duringheating up to and holding at approximately 1700 F. of Type 304 stainlesssteel for degassing, some adverse intergranular carbide precipitationhas been noted; however, Type 304L-an identical composition other thanfor a lower carbon content-has given consistently good bonding resultswith no manifestation of damaging carbide precipitation and hencerepresents a superior selection.

A mass of the selected ferrous metal is shaped in the form of anappropriate billet. Being an extrusion procedure, the present processspecializes in the production of the joints in configurations ofindividual rods, tubes, and like elongated members of generally uniformaxial crosssection. For such products, each billet is best of an axialcross-section roughly similar in shape to that of the desired productbut of several-fold larger area. Thus, for a rod the billet is normallya solid cylinder, while for tubes a thick-walled, hollow, open-endedcylinder is in order. It is advantageous for the rear axial extremity ofthe ferrous billet to be axially symmetrical-say cropped in an axiallyperpendicular plane-to facilitate subsequent mating with a zirconiumbillet. In particular accordance with the present invention, suchferrous metal can and billet are thoroughly degassed by heating over anextended period while continuously drawing a high vacuum upon it. Thetemperature is preferably at least as high as, or higher than, any to beused throughout the extrusion operation. Normally, though, extrusiontemperatures are limited to maximum values somewhat below 1710" F., themelting point of the 17% iron-83% zirconium eutectic; accordingly, aferrous degassing temperature of the order of 1700 F. is particularlywell suited. The higher the vacuum, the better; a vacuum below a smallfraction of a micron of mercury absolute pressure is particularlyfavored. It is beneficial to continue the heating and dynamic evacuationuntil a substantial equilibrium is obtained, with virtualy no furthertraces of gas evolving. A degassing period approximating 3 to 4 hours isfrequently ample. Similar hot degassing of the zirconium metal, though,is inadvisable; the heating, rather than serving to eliminate residualnitrogen, would tend to promote its nitriding with the zirconium. By thesame token, the degassing of the ferrous metal should well be out of thepresence of the zirconium billet, to avoid capture of the freed nitrogenby the gettering action of the zirconium. All degassing may well beconducted in a conventional vacuum furnace or heated autoclave.

The degassed ferrous components are then permitted to cool, whereuponthe vacuum may be broken, preferably with a noble gas; helium isparticularly preferred. Alternatively, it is acceptable to break thevacuum by admitting air although the billet should well not be reheatedwhile in contact with the air. The ferrous billet is thereupon axiallyaligned with a predominantly-zirconium metal billet having much the sameaxial cross-section; the front axial extremity of that zirconium billetshould preferably mate closely with the abutting rear face of theferrous billet, to avoid burdening the extrusion with large interfacialgaps remaining to be closed. To facilitate evacuation of gases from thebillets for the extrusion, it is convenient to dispose the alignedbillets in a closelyfitting vacuum-tight malleable metal can providedwith a sealable evacuation aperture, e.g., a tube or nipple. Not onlyWill such a metal can generally extrude with the billets as a thinenveloping sheath, but upon selection of a relatively soft metal for thecan, such as mild steel, the can will further serve as a suitableplastically deformable material appropriate for easing the passage ofthe billets through the extrusion die. In instances where the degassingvacuum has been broken by noble gas or other protective atmosphere, allbillet alignment and canning operation can profitably and convenientlybe effected in the same atmosphere.

The aligned billets are thereupon evacuated of contacting gas; in caseswhere the billet pair is canned, a high vacuum is drawn through theevacuation aperture therein whereupon the aperture is sealed. Again, thehigher the vacuum, the better; evacuation to a small fraction of amicron or lower is favored. The evacuation billet pair is then heated toextrusion temperature. The range of 1500 to 1650 F. is preferred.Temperatures much higher are better avoided in order not to chanceexceeding the aforementioned 1710 F. eutectic melting point.Temperatures much lower than 1500 F. become disadvantageous becausestiifening of the metals, especially the stainless steel or otherferrous component, with decreasing temperatures would necessitateinordinately high extrusion force. 1600 F. is the apparent optimum.

The heated billet pair is thereupon placed into an extrusion pressbillet chamber-an elongated massivewalled tube of uniform cross-section,normally cylindricalclosed by a die defining an aperture ofsubstantially smaller cross-sectional area. By advancing a close-fittingram into the opposite open extremity of the billet chamber, the billetpair is then promptly hot extruded, with the ferrous metal leading andthe zirconium metal following; upon so being axially pressed from thechamber through the die, the metals form a long, continuous shape of thesame cross-section as the die opening and emerge enveloped in a thinsheath of the can material. To facilitate streamlined flow of the metalinto the die aperture, the die should well define an approximatelyconically converging surface from chamber to aperture. Cross-sectionalarea reduction ratios between billet and die aperture of the order of :1to 6:1 and up to as great as 10:1 have been found satisfactory withlittle difference in resulting bond strength. Reduction ratios as low as4:1 to 2:1, though, evidently sacrifice some bond strength. 6:1 is theapparent optimum. Extrusion speeds produced by ram velocities from aslow as 13 inches to as high as 55 inches per minute have afforded goodresults, suggesting that the choice of speed can be made consistent withthe practical speed range limitative of the particular extrusion pressemployed. At very slow velocities, it is often beneicial to preheat theinterior of the billet chamber to minimize the rate of heat loss fromthe slow-moving billet pair to the chamber walls, ram, and die.

As the resulting extrusion issues from the die, no significantdifference in bond strength has been found to obtain between cases wherethe eflluent extrusion is water quenched as it passes out of the die andwhere it is simply left to air cool quietly. The sheath of can materialis removed by mechanical peeling, if thick enough, or machining and/orgrinding; more conveniently, such materials as soft iron, copper, andbrass may often be quickly and cleanly removed by pickling in aqueousnitric acid, which does not excessively attack zirconium, saidZircaloys, or said stainless steels.

The extremities of the desheathed extrusion may then be cropped or cutstraight and square, thereby providing, as the ultimate product, amember of uniform cross-section featuring an extremity constitutedsolely of the zirconium metal, the other extremity constituted solely ofthe ferrous metal, and a sound metallurgical bond therebetween. Theproduct may be made of virtually any overall length, beyond thatrequired for the juncture, as admitted by the capabilities of theparticular extrusion press by simply resorting to billets ofcommensurate length. Thus, if a long stainless steel rod joined to along zirconium rod is desired, the instant process not only serves toeffect the joining but simultaneously to fabricate the two rods. On theother hand, individual short couplings-say only a few diameters inlength-can be produced; inasmuch as the zirconium metal is readilyfusion welded to like zirconium metal and the ferrous metal to likeferrous metal, an elongated member of each metal may be welded, eitherin the shop or in the field, to its respective end of such a coupling toaccomplish their connection.

In the extrusion product obtained, the area of bonded juncture generallyassumes the configuration of a rather precise conical surfacesymmetrical with the extrusion axis and apexed toward the leadingextremity of the extrusion. With extrusions produced from billet pairsabutted across a planar interface perpendicular to the extrusion axis,the length of the resulting conical juncture is generally several timesthe radial thickness of the metal in which it is located; a junctureslope of 1:10 is typical for such cases. Provided there is no pressingrequirement for maintaining the length of juncture as short as possible,however, the area of juncture over which the disruptive force isdistributed may beneficially be sizeably enlarged by resorting to abillet interface in the configuration of an axially symmetrical conicalsurface with apex in the direction of the leading extremity. Uponextrusion, the resulting conical juncture obtained is proportionatelylonger and hence more expansive. Representatively, a billet interface ofconical surface angled 35 from the longitudinal axis has producedjuncture slope of the order of 1:30 to 1:35, 30 produced 1:35 to 1:40slope, and 25 a 1:45 to 1:50 slope. In addition to increasing generallythe cumulative strength of joined area, such elongation of the junctureaffords a second beneficial result. That is, upon extrusion with theflatter billet interfaces, the extrusion tends to sink somewhat into thefeathered edge of extruded ferrous metal at the outer periphery of thejuncture, and, in the case of tubes, into the feathered edge ofzirconium metal around the inside periphery. Upon varying the billetinterface angle with the axis in 15 steps starting with 90, it has beenfound that the sinking of the can becomes monotonically more slight, themore acute the angle, thus mitigating the extent of the annulardepression which might be left, upon removal of the sheath, as adiscernable imperfection in the otherwise smooth surfaces. Typically, a30 angle for the conical billet interface is evidently adequate forlargely eliminating all such depression, without need for resort to anymore acute angle. A propos where joint length must be minimized, use ofa sawtooth cross-sectioned billet interfacedefining a multiplicity ofconcentric sharp-edged ridges and valleys in the mating faces of eachbillet-beneficially affords not only expanded area, but acircumferentially-corrugated keyed configuration of juncture forenhancing its strength notably against axial tensile stress.

It has been found essential that the ferrous metal precede the zirconiumin the extrusion operation. Reversal of the sequence has provengenerally inoperative. With a planar billet interface, attempts atextruding Zircaloy-2 ahead of Type 347 stainless steel have resulted ingross void formation at the area of intended juncture. Resort toforward-apexed conical billet interfaces has been unavailing; so acutean angle as 20 was necessary merely to eliminate gross voids, but stillleft much of the interface abutted but unjoined.

As an additional special aspect of the present invention, a thin barrierlayer of specifically titanium or niobium may be incorporated betweenthe abutting faces of the ferrous and zirconium billets to complementthe ferrous degassing operation in promoting bond strength. A foil ofeither titanium or niobium of the order of 5 mils in thickness andshaped to conform to the mated interface is normally sufiicient; it isWell to degas the barrier along with the ferrous billet beforeextrusion. Upon extrusion,

the barrier follows the tapered contour of the juncture, representing anunbroken transition layer at the bonded interface. In some instances,use of such barrier layers affords somewhat greater unit tensilestrengths of bond; however, the resulting joints appear more susceptibleto corrosive attack. For example, after several days to one weekimmersion of typical joints in pressurized water at approximately 300 to360 C., oxide formation and some spalling became discernable at theinterface periphery. Superiority in bond strength is believed ascribableto the function of the layer in serving to bar any residual nitrogen,still remaining in the ferrous billet after degassing, from readilydiffusing into the zirconium, while at the same time sustaining soundbonding of its surfaces to both the ferrous and zirconium metals.Empirical investigation evidences that titanium and niobium areextraordinary in accomplishing such dual function; witness: moylbdenumsheet, niobium-vanadium alloy sheet, molybdenum sheet plus niobium sheet(Mo next to stainless steel billet) all have exhibited outright failurein bond ing, while the results with certain other layers were at bestuncertain or anomalous. In any event, sound bonds obtain with thetitanium and niobium, such that use of these specific interlayers, atleast, represents a valuable optional procedure aiming toward enhancedbonding in those situations where the possible compromise of corrosionresistance is acceptable.

Representative apparatus and articles for conducting the present processare illustrated, along with resulting product embodiments, in theappended drawings.

In the drawings,

FIG. 1 is a cross-sectioned elevation of the billet chamber region of anextrusion press including a canned pair of billets ready for extrusion.

FIG. 2 is a partially diametrally-sectioned view of a rod, sans sheath,resulting from tandem extrusion of the FIG. 1 billet pair.

FIG. 3 is a cross-sectioned elevation of such a billet chamber regionshowing a joint of tubular configuration in the course of extrusion.

FIG. 4 is a partially diametrically-sectioned view of a tube, sanssheath, resulting from completion of extrusion of the FIG. 3 tubularjoint.

FIG. 5 is a diametrally-sectioned view of a canned billet pair featuringa barrier layer disposed therebetween.

FIG. 6 is a partially diametrally-sectioned view of a tube, after sheathremoval, featuring a bonded barrier layer at the juncture.

FIG. 7 is a cross-sectioned elevation again of a billet chamber regionshowing a rod featuring a multiplicity of successive joints in thecourse of extrusion.

Referring to FIG. 1, a thick-walled, reentrant, horizontal,hollow-cylindrical extrusion container inner liner l, defining as itsinterior a billet chamber 2, is disposed snugly within a thicker-walled,re-entrant, horizontal, hollow-cylindrical extrusion container outerliner 3, in turn disposed within a massive extrusion container 4.Coaxial with the billet chamber 2, a conically-convergent,circular-annulus die 5, backed by a bolster 6, and fitted tightly in therecess of a die holder 7, is urged against one extremity of the billetchamber 2, by a massive die head 8. The extrusion container 4 and diehead 8, as well as the die holder 7, are stationarily anchored in astructural frame (not shown) and locked in place thereto by acceptanceof locking pins (not shown) in receptacle wells 9, 9', and the tightfitof a thrust-transmissive shim ring 10 located between the die head 8 andsaid frame. Into the opposite extremity of the billet chamber 2, aclose-fitting, solid, cylindrical ram 11, extends, wherein it iscentered by a sliding fit through a centering ring 12. Within theextrusion chamber 2, is positioned a heated tandem extrusion billet paircomprising a solid, right cylindrical mass of hot degassed ferrous metal13, at the fore, followed by a predominantly-zirconium billet 14, aft;both billets are sealed in a vacuum-tight malleable metal can 15, fromwhich gas has been substantially evacuated through an evacuation tube16, which has been subsequently sealed by pinching flat. In operation,the ram 11 is advanced into the billet chamber 2, to force the cannedbillet pair 13, 14, through the die 5.

The FIG. l operation results in a simple, elongated rod sheathed in themetal of the can 15. After removal of that sheath, the produced rod,referring now to FIG. 2, comprises a leading length of the ferrous metal21, into the rear extremity of which a trailing length of thepredominantly-zirconium metal 22, penetrates in the form of a slenderspire. At the axially-symmetrical, rather precisely conical juncture 23,the two metals have becomc soundly bonded metallurgically.

Referring to FIG. 3, the system described in FIG.1 is modified toproduce tubular joints by substituting a different, axially-bored ram31, extending into the extrusion chamber inner liner 1; reciprocablysupported from within the bore of the ram 31, a slender, solidcylindrical mandrel 32, extends in spaced relationship through theaperture of the die 5, and on forward beyond the die head 8. A heatedtandem extrusion billet pair comprising a leading mass of degassedferrous metal 33, and a following mass of predominantly-zirconium metal34-which had, at the outset, been disposed within the extrusion chamberinner liner 1, in the configuration of thick-walled, hollow, rightcylindrical billets sealed in an annular, evacuated, malleable metal can35, slidably impaled upon the mandrel 32-are progressively becomingsqueezed through the die 5, by the steady powered advance of the ram 31,into the configuration of an elongated tube 36, covered inside and outwith a sheathing 37, of the malleable metal of the can 35, pressedfirmly about the mandrel 32. The tube resulting from the FIG. 3operation, after removal of the sheathing 37, comprises, referring nowto FIG. 4, a leading length of the ferrous metal 41, and a trailinglength of the predominantly-zirconium metal 42, soundly bonded togetherat an axially-symmetrical, forward-apexed frusto-conical juncture 43.

Referring to FIG. 5, a more intricate billet pair adapted to improveextrusion of a connected rod comprises a forward, horizontal,cylindrical billet of degassed ferrous metal 51, defining anaxially-symmetrical cavity 52, in its trailing extremity. Spacedslightly apart therefrom, a trailing, solid, coaxial cylindrical billetof predominantlyzirconium metal 53, defines as its forward extremity anaxially-symmetrical spire 54, of shape mating closely with the cavity52. Within -the interspace therebetween is disposed a thin barrier layerof niobium or titanium metal 55, of conforming conical configuration.All are sealed in a vacuum-tight malleable metal can 56, from which gashas been evacuated through an evacuation tube 57, subsequently sealed bypinching. Casual gaps 58, 58', between the billets and can walls providesufficient spacing for gas egress to permit thoroughgoing evacuation.Upon extrusion, this billet pair is adapted to produce a conicaljuncture considerably longer and more acute than in FIG. 2, andcontaining a thin interlayer of the titanium or niobium soundly bondedbetween the two principal metals.

A billet pair (not shown) similar to that in FIG. 5, but constitutedrather of thick-walled hollow cylindrical billets and a correspondinglytruncated barrier layer is adapted to produce a corresponding tubularjoint. Referring to FIG. 6, in such a tube connection, thefrusto-conical interlayer 61, of titanium or niobium, is soundly bondedto the forward ferrous metal mass 62, and to the trailingpredominantly-zirconium mass 63.

In FIG. 7, a composite billet comprising a multiplicity of alternatediscs of degassed ferrous metal 71, 71, and predominantly-zirconiummetal 72, 72', appropriately disposed in an evacuated malleable metalcan 73, is being extruded into a sheathed rod 74, comprising alternatelengths of the ferrous metal 75, 7S', and zirconium metal 76, 76', whilethe resulting bonding between trailing ferrous faces and leadingzirconium faces is sound, the junctures between trailing zirconium facesand leading ferrous faces are very poorly bonded, if at all. Thedesheathed rod may be cut apart to provide a series of nipples, eachbearing one of the well-bonded junctures, while the alternate unsoundjoints are discarded. In this way, a goodly number of sound joints canbe extruded in a single operation, and the resulting composite rodrepresents a cartridged or nested supply of nipples severable as needed.

Further illustration of the quantitative aspects and preferredconditions and procedures of the present method and product is providedin the following specific examples. In Example l, two comparative tandemextrusions of rods are outlined, one without hot degassing of theferrous component, and one with degassing in accordance with the presentinvention. In Example 2, a succession of comparative tandem extrusions,all in accordance with the present invention, of rods demonstrates theeffect of varying different individual process parameters upon theobtaining bond strength. In Example 3, production of tubular joints isexemplified, while in Example 4, the integrity of resulting joined tubesunder internal hydraulic pressure is shown. Finally in Example 5, theresistance of resulting joints to the corrosive effects ofhigh-temperature pressurized water is assessed.

EXAMPLE 1 A right cylindrical billet of Type 304 stainless steel and asimilar billet of Zircaloy-Z were inserted end-to-end into aclosely-litting 16 gage mild steel can 2 inches in outside diameter. Gaswas evacuated from the assembled can to a vacuum of 1 micron. The cannedbillet pair was heated to 1600 F. and promptly extruded, stainless steelend first, through a 0.833 inch circular-apertured die, employingextrusion apparatus generally corresponding with that illustrated inFIG. 1, at ram speed of 13 inches per minute. After quietly cooling inair to approximately room temperature, the resulting extruded rod Wasstripped of the obtaining mild steel sheath, and several axiallyperpendicular cuts Were made through the region of conical juncture toyield several 5716 inch thick discs. Each disc comprised an outer ringof stainless steel surrounding an inner disc of zirconium metal. Twostainless steel studs were welded at diametrically opposite locations tothe ring periphery, and two radial saw cuts were made through the ringapproximately 1/8 inch above and below the stud. In each case, thestrength of the bonding per unit bond area between the meltals wasdetermined by pulling the studs outward with progressively increasingtensile stress and noting the minimum tensile stress needed to rupturethe ferrous-zirconium bond. For comparison, the entire operation wasrepeated with the exception that the ferrous billet and the mild steelcan were held at 1700 F. under a dynamic Vacuum which reduced pressureto 1 micron for approximately 4 hours, whereupon they were permitted tocool under vacuum, and then the vacuum was broken by admitting air, allbefore insertion of the billets into the can. The minimum values oftensile stress which ruptured the ferrous-zirconium bond were asfollows:

P.s.i. First run (without hot degassing) 6,000 Second run (with hotdegassing) 53,000

EXAMPLE 2 In a succession of runs, a multiplicity of connected rods wereindividually produced by hot tandem extrusion in general accordance withthe procedure of the second run of Example 1, including hot degassing ofthe ferrous metal billet and can before nal billet assemblage in eachcase. The diameter of each canned billet pair was nominally 2 inches.Among the runs, different extrusion conditions were employed as a meansfor evaluating the effect of each principal process parameter uponbonding strength. In some instances in the tensile strength tests, themetal parted at the ferrous-zirconium juncture while in others it failedelsewhere, principally at the weld between a stud and the stainlesssteel ring. Particular operating conditions andthe ultimate imposedstresses reached upon metal failure in representative runs are set forthin Table 1 below.

10 Table 1.-Eject of Derent Parameters Upon Stainless Steel-Zircaloy-2Bond Strength Type of Extru- Reduc Extru- Stainless sion tion sion RodExtru- Bond Strength Steel Teru- Ratio Speed Cooling sion (1,000 p.s.i.ten- (AISI perature (areal (in./ N o. sion) type) F.) area) min.)

EFFECT OF TEMPERATURE 347 1, 500 6:1 13 air 23156 48, 27, 48 48, 36, 48347 l, 600 6:1 13 alr 23110 37, 34, 40, 51 347 1, 600 6 1 13 air- 2326114, 38, 3 347 1, 600 6:1 13 air 23262 51, 40, 53 304L 600 6:1 13 air23112 32, 48, 37 304L 1, 500 6 1 13 air. 23263 40, 40, 35 304L 1, 6006:1 13 air- 23264 53, 43

EFFECT OF REDUCTION RATIO 347 1, 600 6:1 55 air 23123 54, 33, 57,

33, 60 304L 1, 600 6: 1 55 air. 23268 38, 48, 40 347 l, 600 10: l 55air- 23163 55, 35, 304L 1, 600 10:1 55 air 23272 36, 32, 56

EFFECT 0F EXTRUSION SPEED 347 1, 600 6:1 13 air 23110 37, 34, 40,1

5 347. 1, 600 6:1 13 a1 23261 14, 38, 35 7 6:1 13 air. 23262 51, 40, 536:1 13 air. 23112 32, 48, 47 6:1 13 air. 23263 40, 40, 35 6:1 13 air23264 51, 53, 43 6 1 55 air 23123 54, 33, 57, 33, 60 6: 1 55 air. 2326713, 35, 57 s; 1 55 air. 2325s 38,421,411

EFFECT OF TYPE OF STAINLESS STEEL AND ROD CO OLIN G 304 1, 600 6:1 13air 23158 54, 51, 53, 48 304 1, 600 6:1 13 Water- 23125 6, 29, 3 6:1 13air 23112 32, 48, 37 6:1 13 aia--- 23263 40, 40, 35 6:1 13 air. 2326451, 53,43 s z 1 13 waren 23125 49, 40, 52 6:1 13 Water 23266 32, 38, 396:1 13 ail 23113 13, 23,9 6: 1 13 water.. 23127 43, 26, 39 6:1 13 air23110 37, 34, 40, 51 6: 1 13 air- 23261 14, 38, 35 6: 1 13 air- 2326251, 40, 53 6:1 13 Water- 23265 49, 60 50 EFFECT OF INTERLAYERS (included5 mil niobum 35, 48

interlayer) 347 1, 600 6:1 13 l air 23116 13,111,211

(included 5 mil titanium interlayer) Footnotez Metal parted elsewherethan at ferrous-zirconium interlayer (principally at Weld between studaud stainless steel).

EXAMPLE 3 A set of live tubulal joints were individually extrudedemploying a system in general accordance with FIG. 3, commencing in eachcase with one hollow cylindrical billet 3%. inches outside diameter and11/2 inches inside diameter of Type 347 stainless steel and one ofZircaloy-2 respectively and a 16 gage mild steel annular can. Thestainless steel billets and cans were first degassed in the mannerdetailed in Example l. In every case the stainless steel-Zircaloyinterface was of frusto-conieal shape with conical surface angled 30 tothe longitudinal axis of the billet pair and apexed toward the stainlesssteel billet. Extrusion conditions were 1600 F. extrusion temperature,approximately 6:1 area reduction ratio, 13 inches per minute extrusionspeed, and air-cooling after eX- trusion. After stripping the steelsheathing from the resulting extruded tube, longitudinal strips about1/2 inch wide were cut from three of the tubes and subjected to varioustests to characterize their bond strentgh. Two

1 1 strips from each cut tube were pulled in tension. None of the stripsfailed preferentially along the joint. In all cases, the strip neckedand fractured either across the all- Zircaloy section or across thejoint near the all-Zircaloy section. Single strips from two of the threeextrusions were used for bend tests. These strips were bent to form acomplete loop over a 2-inch mandrel and no separation occurred at thepoint. Single strips from two of the three extrusions were also rolledin the longitudinal direction to give about a 13% elongation. No failureof the joint resulted from rolling. These strips were subsequentlypulled in tension and the fracture was across the all- Zircaloy section.Stud pull tests normal to the interface were performed on strips fromone of the three extrusions. These gave values of 17,000, 23,000 and41,000 p.s.i. These values are somewhat lower than the general levelobtained with rod extrusions in Example 2, but the geometry of thesections from the strips was such that cuts around the stud had to bemade through the joint manually with a hacksaw; such inelegant cuttingis, of course, quite severe and probably mechanically separated thejunction layers somewhat around the sawed periphery.

EXAMPLE 4 To one of the remaining uncut joints from Example 3, which isapproximately 2 inches in outside diameter with an approximately 1A cutwall thickness, was welded end caps; the resulting closed container wasfilled with Water through a pressure nipple provided, and subjected toprogressively increased internal hydraulic pressure, with cappedextremities essentially unrestrained from axial motion. At 18,900 p.s.i.pressure (representing a hoop stress approximating 78,000 p.s.i.) a 1,46inch bulging of the all-Zircaloy section of tubing was detected, butwith the juncture remaining quite intact. The test was thereuponterminated without actually consummating the apparently inevitableblow-out of the all-Zircaloy wall.

EXAMPLE 5 One of the remaining uncut joints from Example 3 was immersedin pressurized water at 360 C. for 28 days, without indication ofsignificant corrosion attack at the juncture or elsewhere.

It is estimated that the Example 5 test was equivalent in severity to 8years of exposure in 250 C. waterthe realm of interest forpower-productive nuclear reactor service. Further preliminary corrosiontests in aqueous nitric acid have shown no significant preferentialattack at the joint.

As defined in the Metal Handbook, 1948 ed., edited by T. Lyman, pp. 307,554, American Society for Metals, 1948: SAE-1015 steel is a carbon steelcomprised of iron together with the following minor constituents insubstantially the indicated proportions by weight-caribou 0.13 to 0.18%,manganese 0.30 to 0.60%, phosphorus 0.040% (maximum), sulfur 0.050%(maximum). Likewise, American Iron and Steel Institute Nos. 304, 321,and 347 stainless steels are there defined to be austenitic stainlesssteels comprised of iron togther with the following minor constituentsin substantially the indicated proportions by weight:

No. S04-carbon 0.08% (maximum), chromuim 18.0 to 20.0%, nickel 8.00 to11.00%, manganese 2.00% (maximum).

No. 321-carbon 0.08% (maximum), chromium, 17.0 to 19.0%, nickel 8.00 to11.00%, titanium at least 5 times the carbon percentage.

No. 347-carbon 0.08% (maximum), chromium 17.0 to 19.0%, nickel 9.00 to12.00%, columbium 10 times the carbon percentage.

As defined in Alloy Digest, p. SS-55, Engineering Aliloys Digest, Inc.,No. 304-L stainless steel is essentially the same as No. 304, supra withthe sole significant exception that the proportion by weight of barbonis more rigorously limited to 0.03% (maximum). As defined 12 inMetallurgy for Engineers, J. Wolff et al., p. 60, Wiley, 1952, mildsteel is regardable as simple steel containing less than 0.30% by weightcarbon and less than 1.0% silicon and manganese.

Although this invention has been described with particular emphasis uponjoining simple rods and tubes, it is inherently of much widerapplicability. Other extrudable sections, for examplerectangular-sectional conduit, I-beams, tubes featuring internal and/orexternal ribs, straight or spiral, and the like may be joined. Otherextrusion techniques can be applied; a piercing mandrel procedure may beapplied instead 0f the described floating mandrel style of tubeextrusion. Impact, rather than press, extrusion may be in order. Thesaw-tooth and conical interface configurations for the billet pairsuggest varieties of other extended surfaces-eg., threaded spindle andsocket-for enhanced keying of the juncture against axial tension.Extension to other materials, especially to heavy metals, similarlydifficult to join metallurgically is promising. Diverse additionalapplications of the hereinbefore-disclosed methods and products willbecome apparent to those skilled in the art. It is therefore to beunderstood that all matters contained in the above description andexamples are illustrative only and do not limit the scope of the presentinvention.

This patent application is a continuation of our copending patentapplication Serial No. 31,786, filed May 25, 1960, now abandoned,likewise entitled Process and Product of Metallurgically JoiningZirconium to Ferrous Metal, in the names of Joseph Lester Klien, AlbertR. Kaufmann, and Paul Loewenstein.

What is claimed is:

1. A method of metallurgically joining a metal constituted predominantlyof zirconium to a predominantly ferrous metal which compises arraying intandem a mass of said predominantly ferrous metal followed by asubstantially contiguously abutting mass of said predominantly zirconiummetal both Within a substantiallyvacuum-tight malleable ferrous metalcan, and, upon establishing and while maintaining substantial evacuationof gas from the interior of said can, hot extruding said enveloping canand concomitantly, in axial tandem, the therein contained said ferrousmass followed by said continguously abutting predominantly-zirconiummass, such arraying and extrusion being effected after first degassingsaid malleable ferrous metal can and all other metal which is affordedcommunication with said zirconium metal during hot extrusion andcontains nitorgen subject to additional substantial evolution in vacuoat the temperature of extrusion, said degassing being effected byprotractedly heating without melting and conjointly drawing asubstantial vacuum thereupon.

2. The method of claim 1 wherein an intermediate layer of metal selectedfrom the group consisting of niobium and titanium is placed between theabutting surfaces of said masses of predominantly zirconium andpredominantly ferrous metals so that said intermediate layer ismaintained between said masses throughout the extrusion step.

3. A metallurgically bonded point between predominantly zirconium andpredominantly ferrous metals produced by the process of claim 1.

4. A method of metallurgically joining a metal comprised predominantlyof zirconium to be predominantly ferrous metal which comprises degassingsaid predominantly ferrous metal and a ferrous can of size accommodativeof said masses in tandem, by heating said predominantly ferrous metaland said can for a period of 3 to 4 hours at substantially 1700 F. andconjointly drawing a substantial vacuum upon same, thereafter arrayng intandem the resulting degassed predominantly ferrous metal followed andcontiguously abutted by said mass of zirconium both contained insubstantially-vacuumtight envelopment by and within said ferrous can,and, upon establishing and while maintaining substantial evacuation ofgas from contact with said masses in the interior of said can, hotextruding said enveloping can, and concomitantly, in axial tandem, thetherein-contained said predominantly ferrous metal followed by saidcontiguously abutting predominantly zirconium, said extrusion beingcarried out after said masses are heated to the range of 1500 to 1650 F.and effecting a crosssectional area reduction between the canned billetand die aperture of ratio within the range of 5:1 to 10: 1.

References Cited in the le of this patent UNITED STATES PATENTS Re.3,744 Shaw Nov. 23, 1869 1,569,954 Donaldson et al Jan. 19, 19261,771,620 Ehrmann July 29, 1930 2,023,498 Winston Dec. 10, 19352,254,516 Farr Sept. 2, 1941 2,932,885 Watson Apr. 19, 1960 2,932,887McCuaig et al Apr. 19, 1960 14 2,975,893 Johnson Mar. 21, 1961 2,986,273Bandgett May 30, 1961 FOREIGN PATENTS 744,313 Great Britain Feb. 1, 1956OTHER REFERENCES

1. A METHOD OF METALLURIGICALLY JOINING A METAL CONSTITUTEDPREDOMINANTLY OF ZIRCONIUM TO A PREDOMINANTLY FERROUS METAL WHICHCOMPRISES ARRAYING IN TANDEM A MASS OF SAID PREDOMINANTLY FERROUS METALFOLLOWED BY A SUBSTANTIALLY CONTIGUOUSLY ABUTTING MASS OF SAISPREDOMINANTLY ZIRCONIUM METAL BOTH WITHIN A SUBSTANTIALLYVACUUM-TIGHTMALLEABLE FERROUS METAL CON, AND, UPON ESTABLISHING AND WHILEMAINTAINING SUBSTANTIAL EVACUATION OF GAS FROM THE INTERIOR OF SAID CAN,HOT EXTRUDING SAID ENVELOPING CAN AND CONCOMITANTLY, IN AXIAL TANDEM,THE THEREIN CONTAINED SAID FERROUS MASS FOLLOWED BY SAID CONTIGUOUSLYABUTTING PREDOMINANTLY-ZIRCONIUM MASS, SUCH ARRAYING AND EXTRUSION BEIGEFFECTED AFTER FIRST DE-